Method of finishing piezoelectric crystals



TALs

K. B. ROSS March l1, 1947.

HETHOD OP FINISHING PIEZO-ELECTRIC CRYS 3 Sheets-Sheet l Filed Nov. 25, 19455 March 11, 1947. K. e. Ross METHOD OF FINISHING PIEZG ELECT Filed Nov'. 25, 1943 mun d RIC CRYSTALS 3 Sheets-Sheet 2 mafneys ELECTRI C CR' STAL I5 Sheets-Sheet 3 K. B. ROSS Filed Nov. 25, 1943 METHOD OF FINISHING PIELO March 11, 1947.

Patented Mar. l1, 1947 METHOD OF FINISHING PIEZOELECTRIG CRYSTALS Kenneth B. Ross. Chicago, lil. Application November 25, 1943, Serial No. 511,125

(Cl. 17k-327) 1 Claim. 1

My invention relates to resonators in the form of piezo-electric crystals. It has more particularly to do with a process or method of bringlng large quantities of such cyrstals to a, size such that within permissible tolerances they will all have a given oscillation frequency.

In general, it is the object of my invention to produce large quantities of piezo-electric crystals, all of which will oscillate at a desired frequency.

Thus far I have successfully applied my invention in the production of resonators cut from quartz crystals.

In the practice which I have made of my invention. I out slabs of more or less uniform thickness from natural quarta crystals having piezoelectric properties. These slabs are so cut as to have dimensions slightly in excess of those necessary to the vibration of the crystals at the desired oscillation frequency. I have found it expedient to lap these cut slabs to bring their faces into parallelism and to approximate more nearly the dimensions required oi. the finished resonators.

The oscillation frequency of each cut and roughly lapped slab is then measured, preferably by the use of a frequency meter, whose indications correspond with the differences in frequency between` the cut and roughly lapped slab and the oscillation frequency of a carefully prepared standard. The indications of the frequency meter correspond with the differences in the frequency of each\ cut and roughly lapped crystal and the standard resonator.

I then move each of the oversize slabs through a chemical etching bath. It would no doubt be desirable to move each slab through this bath for a distance corresponding exactly with the amount of material required to be removed from the slab in order to give it dimensions which would produce a resonator of the desired oscillation frequencyand corresponding with the oscillation frequency of the resonator employed as e, standard in connection with the frequency meter hereinabove referred to. However. 1 have found it more expedient to subject each crystal to a succession of measuring and etching operations. The ilrst etching operation brings the dimensions of the crystal te a point such that its oscillation frequency closely approaches, but does not quite attain, that required of a resonator having the desired oscillation frequency. I then re-measure the oscillation frequencies of the crystals, and then again subject them to a second etching operation. The second etching operation is less effective than the first; it removes less of the material and is, therefore, a more delicate operation. This delicacy may be attained by moving the crystals more rapidly through an etching bath of given potency. I prefer. however, to move the crystals at substantially the same rate, but through 9. more dilute and, therefore, less potent etching bath.

The measuring and etching operations may be repeated as many times as may be desired, each time with a more delicate etching operation than in the preceding etching operation, until each crystal is brought to dimensions which will give it as nearly as may be desired the -oscillation frequency of the given standard.

In practice I have found that four successive measuring and etching operations, with successively more dilute etching baths, will produce,

resonators having oscillation frequencies within the tolerances permitted by buyers' specifications currently in vogue.

The method of my invention is one which can be carried on successfully by relatively unskilled workers and in such a way as greatly to enhance the quantity of satisfactorily nished resonators which can be produced in a given time by a given operator or crew of operators. The details of my invention and of an apparatus which may conveniently be employed in carrying it out are set forth in the following specification. The apparatus is illustrated in the accompanying drawings, in which: A

Fig. 1 is a perspective view indicating in a general way the arrangement of the apparatus;

Fig. 2 is a perspective view of one of the resonator crystals to be produced;

Fig. 3 is a plan view of apparatus which may conveniently be employed for carrying the crystals through the etching bath for appropriate distances:

Fig. 4 is a side view. largely in cross section. on the plane of line l-I of Fig. 3 of this apparatus; and

Fig. 5 is a cross sectional view of this apparatus taken on a plane represented by the arrows 5-6 of Fig. 3.

Corresponding parts have the same reference numerals in the various figures.

Referring first to the apparatus as more generally depicted in the perspective view of Fig. i: il) represents a table on and in which the apparatus may conveniently be arranged. il indicates a frequency meter. It is connected in a circuit containing a resonator I2 having the desired oscillation frequency. A cut and roughly lapped slab of quartz crystal, such as that indicated at i2 in Fig. 2, is inserted into the iig i3.

This jig in essence comprises electrodes such as those with which the crystal will later be associated in use, together with means for clamping the crystal temporarily between these electrodes. The two crystals are thus coupled in similar oscillator circuits, the outputs oi' which are coupled to produce a beat frequency. This beat or difference frequency is supplied to the frequency meter Il. so that its indicator hand will assume a position corresponding with the difference between the oscillation frequency of the standard crystal and that of the slab from which a more or less exact duplicate is to be formed.

The scale of the frequency meter could be arranged to read in cycles per second, but I have found it expedient to number this scale more or less arbitrarily, as, for example, from to 10. Upon this arbitrary scale a reading of "0 indicates that the oscillation frequency of the crystal slab corresponds exactly with that of the standard resonator i2. An indication of i represents a slab which requires but a very slight amount of etching in order to bring its dimensions down to a point where its oscillation frequency willcorrespond with that of the standard resonator. A reading of "2 indicates that twice as much material must be removed from the quartz slab in order that its oscillation frequency shall correspond with that of the standard resonator. The other indications, 3, 4 f 9, 10, indicate that the crystal slabs whose oscillation frequencies are undergoing measurement, require correspondingly increased reduction in dimensions in order to bring their vibration rates into accord with the vibration rate oil the standard resonator i2'. The apparatus and circuits for making this comparison between an unfinished slab and a standard resonator are known and understood to the art and need not be more elaboratcly explained herein.

The milliammeter whose dial is indicated at i4, is included in a circuit which is closed when the unfinished slab of quartz is clamped in the iig I3. and such that the activity of the crystal in the Jig will be reflected by the position assumed by the hand of the milliammeter I4. The initial test circuit, including this miliiammeter, enables the operator at once to discard any crystals whose activity does not come up to the required standard of excellence.

I have found it convenient to locate the etching bath and its concomitant apparatus on or in the same table as that upon which the foregoing measuring instruments are located. The etching bath is a chemical solution, such for example as hydrofiuoric acid, as described in Marrison Patent 1,869,160, issued July 26, 1932, although I have found it expedient, in lieu thereof, to use a bath of ammonium uoride. The bath is preferably placed in a container which is equipped with a circulating pump for maintaining the bath in a condition of more or less uniform effectiveness. The container for this chem ical bath is preferably mounted in a water bath or jacket. whereby a given temperature of the bath may be maintained by thermostatically con trolled electric heating elements. There may desirably be a thermometer il' by means of which the operator can be sure that the requisite temperature is being maintained.

I have found it expedient to provide a conveyor belt for conveying the crystals through tre etching bath and to have them thus conveyed at a uniform speed, and also to provide the bath and conveyor mechanism with a scale to guide the operator in depositing the unfinished crystals upon the conveyor belt at points such that they will be etched in the appropriate amounts or degrees.

I have found that the details of this apparatus may conveniently be arranged as follows: The etching bath I5 of hydroiluoric acid or ammonium fluoride is placed in a tank-like container I6. Within this tank is mounted a conveyor belt I1. This is in the form of three metal sprocket chains mounted side by side in simulation of a single chain. Each of the chains is trained around sprocket wheels, such as those indicated at Illa, l8b and lBc. at the driving end and over corresponding sprocket wheels, such as indicated at ISa. |912 and Iilc at the other end.

The sprocket wheels la, illb and llc are fixedly mounted on a shaft 2B. This shaft carries another sprocket wheel 2l. It is driven by a sprocket chain 22 which is trained around another sprocket wheel 23. This latter sprocket wheel is mounted on a shaft 2l, which. by means of still another sprocket wheel 25 and still another sprocket chain 26, are driven from-anelec tric motor 2l' (Fig. 3). preferably of the synchronous type and is supplied with electric current at uniform frequency in the manner now commonly in vogue by utility companies, and in such a "way that the electric motor and the associated mechanism will run at a reliably uniform speed.

The electric motor is Aswill subsequently appear. the crystal slabs.

which require to be etched, are placed upon the upper part of the conveyor chain. The direction of motor rotation, etc., are such that this upper flight of the conveyor belt will move in the direction indicated by the arrows in Figs. 3 and 4.

A scale 20, graduated from 1 to 10, as best shown in Fig. 4, is located immediately above the level of the etching solution and extends substantially the full length of the chain conveyor I1. A perforated guide IU is suitably secured beneath the upper stretches of the chains comprising conveyor I1 and at the discharge ends of these conveyor chains the guide has an upwardly sloping portion 3i, as best shown in Fig. 4. In order to assure that the chains will rest upon the guide Bil, 3i and not be stretched taut so as to be raised therefrom, a brake is provided on the idler pulleys ila, fsb, and idc. this brake being in the form of a sponge rubber block l! which imposes a frictionai drag on the conveyor chains so that the slack in the chains will be present in the upper stretches thereof.

In order that the etching action of the etching bath may be uniform, I have found it desirable to mount the tank Ii in a water bath or tank, the level of the water being indicated at 8l and the bottom of the water tank at II. Beneath the bottom of this water bath or tank, and preferably within an enclosing box, are mounted three electric heating elements II. Electricurrent is supplied to these heating elements under the control of a thermostat Il, which. through suitable relays contained in a box 42, controls the energization of the heating elements 1I. The operation is such as to maintain a uniform temperature within the water bath and therefore within the etching bath Il A further means for maintaining uniform the' etching action of the etching bath Il, comprises means for circulating the etching solution. This means comprises a rotary pump 42 driven by a motor u, the pump having an inlet I6 adjacent the bottom of the tank I6 and discharging through a pipe IB which extends longitudinally above, and slightly to the side of, the multiple chain conveyor I1, as best shown in Fig. 5. .At intervals along its length. the pipe 48 has nozzle perforations so that a portion of the discharge from the pump is sprayed substantially uniformly along the length of the chain conveyor. The left-hand (Fig. 4) end of the pipe has a Y formation, one end SII of the Y being provided with a stopper 52 which may be removed for cleaning the pipe 48 by means of a, brush. The other portion 54 of the Y extends downwardly to the bottom of the tank I6 and has a return bend portion 5B of reduced diameter forming a nozzle to direct the etching liquid along the bottom of the tank I6 to stir up any sediment which might otherwise collect along the bottom and to provide sufficient circulation to maintain the etching fluid substantially homogeneous throughout the tank.

The tank I6, together with the conveyor driving motor and mechanism. as well as the pump and its motor, are preferably constructed as a f unit so that they may be removed from the table I0. For this purpose the tank IB is provided with a pair of handles B8 at its ends.

In carrying out the method of my invention in quantity production of piezo-electric quartz crystals. the operator inserts the partially finished crystal in the jig I3 and first notes from the reading of the milliammeter Il whether the crystal has the required degree of activity. If it does not have such activity. it is discarded, but if it has sufficient activity it will cause its associated oscillator to oscillate at the inherent resonant frequency of the crystal. By heterodyning the output of this oscillator with the output of the oscillator associated with the standard I2'. a beat frequency, representing the difference between the frequencies of the two oscillators, will be produced and the magnitude of this beat frequency will be indicated by the frequency meter II.

Assuming, for example, that this meter reads 5.5 in a particular case, the operator will then remove the crystal I2 from the iig I3 and place it on one of the chains of the conveyor I1 in alignment with the graduation "5.5" on the scale 29. The crystal will thereafter be carried by the conveyor I1 at a constant speed until lt reaches the 0 position, when it is removed by the operator. It will be noted that due to the inclined portion 3|, the crystal will be substantially removed from the etching solution as it reaches the "1 graduation on the scale 29. There will. however, usually be suicient etching solution adhering to the crystal so that theV etching action continues to some extent until the crystal reaches the unloadlng position at 0.

After this etching operation has been cornpleted, it is repeated a number of times, successively using an apparatus containing an etching solution of less potency, or by moving the crystals at higher speed, or both.

Generally speaking, the potency of the etching solution comprising, for example, hydrouoric acid or ammonium fluoride, is determined not only by the concentration, but also by the temperature. Thus, there are a number of factors which may be varied to reduce the amount of etching: (l) the composition or concentration oi' the etching solution may be changed; (2) the temperature at which the etching solution is maintained may be reduced; (3) the speed of the chain conveyor belt may be increased. I have found that it is most convenient to reduce successively the concentration of the active agent in the etching solution. and also to reduce its temperature in order to obtain the reduced potency or the reduced etching rate for successive stages of the complete' process, since the speed of movement of the chain coveyor may thus be maintained at a constant optimum value corresponding with the average abilities of the operators to make the beat frequency determinations and to place the crystals upon the chain conveyor and to remove them therefrom.

In the quantity production of crystals, I have found it convenient to provide four sets of the etching apparatus such as described herein, so that it will not be necessary for the operator to change the settings of the thermostat or to change the scale of the beat frequency meter II. It will be understood that the beat frequency meters used on the successive stages will have similar appearing scales, but will require geometrically progressing lower beat frequencies to obtain full scale deflection of their indicators.

If, however, the production rate is not sumcient to warrant the acquisition of four sets oi' the complete apparatus, some saving may be eected by using a common table, water bath, and heating apparatus for four sets of the etching bath and conveyor mechanisms, and this is one of the reasons why the latter parts are made removable from the water bath as a unit. Under the latter circumstances. the frequency meter Il will be provided with scale changing means and the scale will be adjusted by the operator to accord with the particular etching bath being used.

After completion of the lapping operations by which the crystal is partially finished, it is found that the surface of the crystal contains microscopic imperfections in the form of partially severed extremely minute fragments. The activity oi.' the crystal has been found to be disadvantageously affected by such adhering fragments. Further, it has been discovered that these fragments are substantially completely removed by etching for a reasonable length of time with a reasonably powerful etching solution. It is partly for this reason that I have found it desirable to rely upon the etching process for a substantial reduction in the thickness oi' the crystal. The word "substantial" is intended to mean "large" in the microscopic sense; that is, the etching should be carried on a sumcient length oi' time to remove the microscopic fragments which are partially severed from the crystal.

From the foregoing, it will appear that the improved method of finishing crystals to cause them to have a predetermined resonant frequency makes it possible for operators of negligible skill quickly and accurately to produce crystals finished to the required standards. since the steps of the process are of utmost simplicity and do not require the exercise of any substantial skill or judgment based on experience. An operator may therefore be trained to perform the method in a, very short time, and will be able to produce more crystals. with less spoilage, and Within a smaller tolerance, than could be produced by a highly skilled operator using the conventional hand-lapping method for reducing the crystals to the required size.

While I have disclosed the preferred manner of carrying out the method of my invention, it will be apparent to those skilled in the art that minor variations end modlcations in the individuai steps thereof may be made without departing from the underlying principles oi the invention. I therefore desire, by the following claim, to include within the scope of my invention such modified and varied methods by which substantially the results of my invention may be obtained through the use ci substantially the same or equivalent steps.

I claim:

The method of finishing quartz piezo-electric crystals to cause them t0 conform closely to a standard in their resonant frequency characteristics. which comprises, determining empirical numbers representative of the frequency diii'er ences between the resonant frequencies oi a plurality of unfinished crystals and that of a standard crystal, and passing such unnished crystals through an etching bath of known etching propcrystal.

KENNETH 44B.4 ROSS.

REFERENCES The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 928,922 Albert July 27, 1909 2,378,052 Waldman et al June 12, 1945 2.364.501 Wolfskill Dec. 5. 1944 

